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Date:   Fri, 11 Nov 2022 10:32:55 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] arm64: ftrace: Define ftrace_stub_graph only with
 FUNCTION_GRAPH_TRACER
Message-ID: <202211111032.BBEEDFA0@keescook>
References: <20221109192831.3057131-1-samitolvanen@google.com>
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On Wed, Nov 09, 2022 at 07:28:31PM +0000, Sami Tolvanen wrote:
> The 0-day bot reports that arm64 builds with CONFIG_CFI_CLANG +
> CONFIG_FTRACE are broken when CONFIG_FUNCTION_GRAPH_TRACER is not
> enabled:
> 
>  ld.lld: error: undefined symbol: __kcfi_typeid_ftrace_stub_graph
>  >>> referenced by entry-ftrace.S:299 (arch/arm64/kernel/entry-ftrace.S:299)
>  >>>               arch/arm64/kernel/entry-ftrace.o:(.text+0x48) in archive vmlinux.a
> 
> This is caused by ftrace_stub_graph using SYM_TYPE_FUNC_START when
> the address of the function is not taken in any C translation unit.
> 
> Fix the build by only defining ftrace_stub_graph when it's actually
> needed, i.e. with CONFIG_FUNCTION_GRAPH_TRACER.
> 
> Link: https://lore.kernel.org/lkml/202210251659.tRMs78RH-lkp@intel.com/
> Fixes: 883bbbffa5a4 ("ftrace,kcfi: Separate ftrace_stub() and ftrace_stub_graph()")
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook